1. Field of the Invention
This invention relates to distributed electromagnetic wave filters and more particularly to ground structures formed in the resonators for planar and folded filters.
2. Description of the Related Art
A distributed element filter is an electromagnetic (EM) wave filter designed for frequencies between approximately 100 MHz to approximately 100 GHz and more typically between 500 MHz and 60 GHz. These bands may be referred to as RF or microwave. At these frequencies, the physical length of passive components is a significant fraction of the wavelength of the operating frequency, and it becomes difficult to use the conventional lumped element model. The distributed element model allows these components to be designed using transmission line theory better suited for these frequencies. These filters may be configured as lowpass, highpass, band pass or bandstop filters.
The filter is made up of one or more coupled resonators. A resonator oscillates at some frequency, called its resonance frequency, with greater amplitude than others. Selection of the number and design of individual resonators determines the nature of the filter response. The resonators may be grounded at either or both ends or left open. The wave may be coupled from one resonator to the next by direct-coupling in which a transmission line directly connects one resonator to the next or by parallel coupling in which the waves are coupled through the dielectric media (air or some other dielectric). The wave may be coupled into and out of the filter by any suitable means including direct-coupling or capacitive coupling. The filter may be configured in any one of many different toplogoies including, but not limited to, interdigital, comb-line, parallel-coupled line, hairpin parallel coupled line, short circuited quarter-wave stub band pass, open circuited quarter-wave stub band stop.
The filter may be configured as a microstrip or a stripline. A microstrip is made up of a conducting strip (the “resonator”) separated from a ground plane by a dielectric layer (air or a dielectric material). A stripline is made up of a conducting strip (the “resonator”) sandwiched between parallel ground planes separated by a dielectric (air or dielectric material). The conducting strip is typically but need not be equally spaced between the ground planes. The dielectric layer typically exhibits a uniform dielectric constant but may vary. The dimensions of the resonators and the cavity are on the order of the wavelength of the EM wave applied to and modified by the filter. For example, the resonators may have a length of one-quarter or one-half the center frequency wavelength (λ).
FIGS. 1a through 1c illustrate an embodiment of a 5-pole stripline bandpass filter 10. Four ¼λ planar resonators 12a through 12e lie in a dielectric layer 14 equally spaced between an upper ground plane 16 and a lower ground plane 18 that define a cavity 20. In this embodiment each resonator has a length “l” and a width “w”. The length “l” generally determines the center frequency of the filter and the width “w” (and thickness) generally determine the impedance. The resonators are connected to ground at one end to a side ground plane 21 and open at the other end. The cavity has a width “a” which is the resonator length “l” plus any additional unoccupied space and a length “s” which is the center-to-center spacing of the resonators. This spacing generally determines coupling of the electric field E 22 (and magnetic field M 24) between resonators. An electromagnetic wave 26 is coupled into the filter via an input 28 and is parallel-coupled from one resonator to the next and is coupled out of the filter as filtered wave 30 via an output 32. The propagation of the wave from one resonator to the next filters the wave according to the designed filter response (e.g. low pass, high pass, band pass or band stop).
However, the total filter response also includes undesirable components due to the coupling of the propagating wave between non-adjacent resonators and due to the propagating of the wave down the waveguide formed by the cavity and external ground planes. For an electromagnetic wave to propagate down a rectangular cross-section shaped cavity with minimal attenuation, the cavity has to be at least a half wavelength wide. If the cavity is less than a half wavelength wide, the wave will still propagate down the cavity but it will be attenuated according to α=(L*27.2875/a)*SQRT(1−εr*(2*a/λ0)2) dB where α is the attenuation for a given length L of transmission line or cavity, L is the length of the transmission line or cavity, λ0 is the wavelength of the wave, εr is the relative dielectric constant of the dielectric material in the cavity and a is the cavity width. For a given filter design the cavity width “a” may be such that the attenuation of the wave travelling down the waveguide and the attenuation of the wave coupled between non-adjacent resonators is not sufficient to effectively eliminate these components from the total filter response. Standard filter design tools assume these undesirable components are zero. Therefore, their existence not only affects the filter response in a negative manner but also in a manner not predicted by the design.
FIG. 2 illustrates the same 5-pole bandpass stripline filter in a “folded” resonator configuration. Resonator 12a (and each of the resonators) is folded in a plane normal to the upper and lower ground planes 16 and 18. An internal ground plane 34 suitably separates the upper and lower planar segments 36 and 38 of resonator 12a, which are connected by a vertical segment 40. The cavity width “a” is largely unaffected as it determined primarily by the unfolded length “l” of resonator 12a. Folding of the resonators is done to reduce the overall footprint of the filter. Vertical folding was proposed, for example by Japanese Patent Laid-Open Nos. 5-152,815/1993 and 5-175,702/1993 and by U.S. Pat. No. 5,621,365.
Folding does however negatively affect the performance of the filter. For an EM wave to be stimulated into a waveguide with width “a” there needs to be a vertical component in the waveguide. In a planar resonator structure there are no vertical components so the EM wave is more difficult to stimulate. Only the imperfections in the physical structure actually produce the unwanted wave. In the folded resonator configuration the vertical segment serves to better stimulate the propagation of the EM wave through the waveguide structure. This in turn increases the undesirable components attributable to waveguide propagation and coupling between non-adjacent resonators.
FIG. 3 illustrates a designed passband response 42 for a 5-pole bandpass filter. The achieved passband response 44 for the planar configuration has a wider “skirt flare” (less attenuation outside the passband) than desired. The achieved passband response 46 for the folded configuration is markedly worse. A designer may have to add resonators (“poles”) to achieve, if possible, the desired response. Additional resonators increases insertion loss, reduces the quality factor and increases the size of the filter.